Scientists at Berkeley Lab have invented multiband gap semiconducting materials for developing solar cells that could achieve power conversion efficiencies of 50 percent or higher.

Description

A single junction of the materials contains three band gaps that together absorb photons from virtually the entire solar spectrum, providing high efficiency without complex, high cost, multijunction fabrication. Layered, multijunction cells are the most efficient photovoltaics currently on the market. A power conversion efficiency of 30 percent has been achieved for the most efficient two-junction cell.

Wladyslaw Walukiewicz, Kin Man Yu, and Junqiao Wu have created highly mismatched alloys (HMAs) by replacing a fraction of group VI atoms in traditional IIVI group semiconductor alloys with oxygen. The alloys are called "highly mismatched" because of the size and electronegativity differences of the component atoms. The Berkeley Lab researchers have demonstrated creating epitaxial II-VI films, specifically ZnMnOTe and CdMgOTe, using ion implantation followed by pulsed laser melting.

A split band gap is created in the Berkeley Lab materials because oxygen is much more highly electronegative than the host metals. In most HMAs the split occurs inside the conduction band, which is not useful for solar cells. In others, a well defined band exists below the conduction band, allowing photons to be absorbed efficiently at three energy levels. The Berkeley Lab scientists have developed a band anticrossing model to predict the split gap effects of various materials. Using the model they successfully predicted that adding oxygen impurities to ZnMnTe and CdMgTe would result in highly efficient materials.

Berkeley Lab inventors have produced p-type and and n-type versions of the splitband material and observed photovoltaic response in a wide photon energy range. Optimal efficiency will be approached as researchers are able to increase the depth of the oxygen layer.

Benefits

Makes possible power conversion efficiencies surpassing 50% with a single p/n junction

Multiband semiconductor compositions for photovoltaic devicesThe highly mismatched alloy Zn.sub.1-yMn.sub.yO.sub.xTe.sub.1-x, 0.ltoreq.y<1 and 0<x<1 and other Group II-IV-Oxygen implanted alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn.sub.1-yMn.sub.yTe host. With multiple band gaps that fall within the solar energy spectrum, Zn.sub.1-yMn.sub.yO.sub.xTe.sub.1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.